The invention was made in connection with the counting and measuring of fish carried with water within a pipe, the counting and measuring being carried out by means of an optical apparatus which comprises a camera and is connected to a computer for analysing camera images. The present description is essentially directed towards this field of application, without the intention of being limiting. The counting/measuring apparatus as such is not part of the invention.
It is known to provide windows in a pipe wall to allow counting and measuring of objects passing. It is often preferred to mount a transparent pipe section in the pipe because that facilitates observation from several sides and because it makes it easier to have the necessary light where liquid and objects are to be observed.
Light refraction leads to the distortion of objects within the pipe when they are observed from the outside, and this is destructive to optical measuring, for example the measuring of the area of the objects or pattern recognition.
In particular, regions next to the centre line of the pipe are subject to distortion.
To avoid said distortion, it is common to use a transparent pipe section with at least one plane transparent side, through which the objects may be observed; typically a rectangular pipe section is used.
There are several drawbacks to mounting a rectangular pipe section to an ordinary circular pipe. A transition between a round and a rectangular cross-section increases the costs, and the dimensions of the pipe section will be unduly large. Further, turbulence or air pockets are apt to occur, interfering with the measurements.